During the past decade flip chip technology has emerged as a popular alternative to wire bonding for interconnecting semiconductor devices such as integrated circuit (IC) dies to substrates such as printed circuit boards, carrier substrates, interposers and other dies.
“Flip chip,” is also known as “controlled collapse chip connection” or its acronym, “C4.” With flip chip technology, solder balls/bumps are attached to electrical contact pads on one face of a die/chip. The flip chip dies are usually processed at the wafer level, i.e., while multiple identical dies are still part of a large “wafer.” Solder balls are deposited on chip pads on the top side of the wafer. The wafer is sometimes “singulated” or “diced” (cut up into separate dies) at this point to provide a number of separate flip chip dies each having solder balls on the top face surface. The chips may then be “flipped” over to connect the solder balls to matching contact pads on the top surface of a substrate such as a printed circuit board or carrier substrate on which the flip chip is mounted. Solder ball attachment is usually provided by reflow heating.
As IC dies have become more complex, the number of solder bumps/balls on flip chips have increased dramatically. Whereas in the past the solder balls were usually provided by relatively large round solder balls attached to the chip contact pads, more recently copper pillars (“CuP's”) have been used in place of the solder balls. A CuP is an elongated copper post member that is attached at one end to a contact pad on the flip chip die. The CuP extends outwardly from the die in a direction perpendicular to the face of the die. Each CuP has a generally bullet or hemisphere shaped solder piece attached to its distal end. The CuP's are bonded by this solder piece to corresponding contact pads on a substrate as by reflow heating. CuP's are capable of being positioned much more densely, i.e., at a “higher pitch,” than conventional solder balls/bumps. One manner of facilitating connection of a substrate to a die having such high CuP density is to provide bond fingers, rather than conventional contact pads, on the substrate to which the flip chip is to be mounted. The bond fingers are elongated contact pads that may be positioned in close parallel relationship without any insulating material between them.